Anatomy of Permanent‎ ‎Mandibular‎ First‎ Molars in a Selected Iranian Population Using ‎Cone-beam Computed Tomography

Introduction: Knowledge of radicular anatomy has a crucial impact on endodontic practices. Since some anatomic features such as modifications of Vertucci are not evaluated adequately, this study was conducted. Methods and Materials: In this in vivo study, cone-beam computed tomography (CBCT) images of‎ 312‎ intact bilateral first‎ molars‎ from‎ 156‎ patients‎ (‎79‎ ‎males‎ and‎ 77‎ females with an average age of ‎35.58‎±‎11.17‎ years‎)‎ were‎ investigated by a trained dentist in terms of number‎ of‎ roots,‎ number‎ of‎ canals‎ in‎ each‎ root‎ and‎ in‎ ‎each‎ tooth,‎ and shapes‎ of‎ canals‎ according‎ to‎ Vertucci’s‎ classification‎ and‎ its‎ modifications.‎ Groups were compared using the Chi-square test. The level of significance was set at 0.05. Results: Of all teeth, 5.2%‎ had‎ 3‎ roots.‎‎ ‎Mesial‎ roots‎ had‎ mostly‎ 2‎ canals‎ while distal roots had a similar frequency of 1 and 2 canals.‎ ‎Of‎ all‎ teeth,‎ ‎‎39.7%‎ had‎ 3‎ canals,‎ 45.2%‎ had‎ 4‎ canals,‎ 13.8%‎ had‎ 5‎ canals,‎ and‎ 1.3%‎ had‎ 6‎ canals.‎ There were no significant differences between males and females, ‎in terms of number of roots (P=0.137), number of canals in mesial (P=0.453) or distal roots (P‎‎=0.328), and total number of canals (P=0.138).‎ The most frequent Vertucci classes in mesial and distal roots were IV ‎‎followed‎ by‎ II and I‎, respectively. There were no significant differences between males and females in terms of Vertucci classes of mesial (P=0.211) or distal (P=0.205) roots. Conclusion: In this population, there were 3 to 6 canals per tooth (mostly 4 and 3 canals).‎ Males and female’s ‎might be similar regarding the number of roots, or number of canals in each root, number of ‎canals in each tooth, or the predominant canal shape in each root.


Introduction
roper knowledge of the internal anatomy of root canals is crucial for a successful endodontic treatment which relies on appropriate cleaning and shaping [1,2]. Since mandibular first molars emerge very early in the mouth and have complex surfaces, they are one of the teeth most requiring root canal therapy; and therefore the knowledge of their root canal anatomy is important [2,3]. These teeth usually have three canals within two roots although they can change due to ethnicity or normal variations [4][5][6][7].
Since ethnic background can affect root anatomy of mandibular first molars, it is important to document properties of these teeth in various populations. However, studies on Iranians are few and controversial [2,3,[8][9][10][11]. Many aspects of anatomic features are not usually covered by them (such as modifications of Vertucci classification which is not covered in many studies worldwide). Many of older studies have used conventional or 2D radiography techniques that can be less accurate than 3D radiography techniques [12]. Nevertheless, recent studies have mainly used cone-beam computed tomography (CBCT) due to its numerous advantages [8][9][10][11].
This study evaluated number and shape of roots and canals of permanent mandibular first molars using CBCT images of an Iranian sample population.
Descriptive statistics were calculated. Groups were compared using the chi-square of Statistical Package for Social Science (SPSS, version 24.0, SPSS, Chicago, IL, USA). The level of significance was predetermined as 0.05.

Results
The observer was calibrated through repeating the diagnosis of cases (especially more difficult cases) under the supervision of a dental anatomist and an endodontist. Of 156 enrolled patients, 79 were males and 77 were females. Patients' average age was 35.58±11.17 years. Of patients, 101 (64.7%) were aged between 20 and 39 years old, while 55 (35.3%) were between 40 and 60 years old. Table 1 summarizes the number of canals. Among 312 assessed teeth, 16 (5.2%) bilateral teeth in 8 patients had 3 roots; in all these cases, the third root was distolingual. All other teeth had 2 roots. Mesial roots had mostly 2 canals and in few cases 3 canals (Table 1). Distal roots showed a rather similar distribution of 1 and 2 canals, in addition to very few 3 canals (Table 1). All distolingual roots contained only 1 canal. Overall, number of canals ranged between 3 and 6. Of 312 teeth, 39.7% had 3 canals, 45.2% had 4 canals, 13.8% had 5 canals, and 1.3% had 6 canals. All 6-canaled teeth were 3rooted. There were no significant differences between males

Anatomy of permanent mandibular first molars
and females, in terms of number of roots (P=0.137), number of canals in mesial (P=0.453) or distal roots (P=0.328), and total number of canals (P=0.138). Tables 3 and 4 present Vertucci classes and Vertucci modifications. In mesial roots, the most common classes were type IV followed by II. In distal roots, the most common class was type I (Tables 2 and 3, Figure 2). The Chi-square did not show significant differences between males and females in terms of Vertucci classes in the mesial root (P=0.211) or distal root (P=0.205).

Discussion
The findings of the present study showed that only 5.2% of first molars had three roots. Our results were similar to other Iranian studies reporting prevalence of third roots ranging between zero and 3.9% [2,[8][9][10][11]. Also our findings were consistent with studies on Jordan [14], India [18], Turkey [19,20] and Sudan [21]. However, they were not in line with results from South Korea [22] and Burma [7]. It seems that presence of distolingual roots might depend on ethnicity, and results in Asians indicate a rather high prevalence of the third root (Table 4) [4-7, 22, 23].
In this study, prevalence of three and four canals in each tooth were rather similar. This finding was comparable to results of Kim et al. [22], Zhang et al. [23], Al-Qudah and Awadeh [14] who reported similar prevalence of three and four canals, but was in contrast to observations of Demirbuga et al . [20], Chourasia et al. [18], Ballullaya et al. [6], Nur [19] and Masoudi [9] who reported a considerably greater prevalence of 3-canaled roots compared to 4-canaled roots. Three-canaled roots can be problematic in endodontic treatments. This research showed prevalence of 3-canaled roots about 13% in mesial roots and 3.5% in distal roots; this was consistent with findings of Al-Qudah and Awawdeh [14], De Pablo et al. [5] regarding mesial roots and Ahmed et al. [21] regarding distal root. However, our result was not in line with other reports, which might be due to differences in ethnicity or methodologies. In this study, most of mesial roots had 2 canals but about 13% of them had 3 canals. This was similar to other studies stating that 2-canaled roots are much more prevalent (summarized in Table 5). In distal roots, similar prevalence of one or two canals were observed in this study (with very few cases of three canals). Such a high frequency of two canals in this root was similar to studies of Ahmed et al. [21], Arjmand et al. [10], and Al-Qudah and Awadeh [14]. All distolingual roots were single-rooted as what was reported by Akhlaghi et al. [2].  [20] CBCT (in vitro) 1748 0.85% 95.8% 2% Chourasia et al./India [18] in vitro (clearing technique) 150 94.6% 5.3% Garg et al./India [24] in vitro (periapical radiography) 1054 6% Al-Qudah, Awawdeh/Jordan [14] in vitro (clearing technique) 330 96% 4% Ahmed et al./Sudan [21] in vitro (clearing technique) 100 94% 3% Mirzaie et al./Iran [11] CBCT (in vivo) 66 100% Ballullaya et al. [6] review of literature of 97   The most common types of canal which were found in mesial roots of this study were Vertucci classes IV (40%), II (21.1%) and VI (8.1%), while about 16% of cases were Vertucci modifications. Higher prevalence of type IV followed by type II was seen in most other studies [2,8] except those reported by Zafar et al. [24] and Masoudi et al. [9] (Table 7). In distal roots, most common Vertucci types were I (43.6%), III (17.6%), and V (15%); about 9% of cases were Vertucci modifications (Table 6). This was consistent with studies of Zafar et al. (37), Arjamand et al. [10], and Masoudi et al. [9]. Although Vertucci classes can simplify reports, they are not sufficient to cover all complexities of root canal structures which were observed in this study and few others that have evaluated Vertucci modifications [7,14]. Sometimes, real canals are much more irregular to be easily categorized into one of Vertucci classes or modifications. Differences might be attributed to ethnicity.

Conclusion
In this population, there were 3 to 6 canals per tooth (mostly 4 and 3 canals). Males and females might be similar regarding number of roots, or number of canals in each root, or number of canals in each tooth. The most frequent Vertucci classes in mesial and distal roots were IV (followed by II) and I, respectively without a significant sex dimorphism.